NBCSports.com's top 5 worst ever MLB trades

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1. A's trade Mark McGwire to Cardinals (1997)

Mark McGwire won the AL Rookie of the Year in 1987 after smacking a rookie record 49 homers. He helped the A's win the World Series in 1988 and was a nine-time All-Star with Oakland but was traded in 1997 to the Cardinals for T.J. Matthews, Eric Ludwick and Blake Stein. While Big Mac averaged 44 dingers during his five years in St. Louis, the players shipped to Oakland just helped raise the team's ERA.

2. Expos trade Pedro Martinez to Red Sox (1997)

Many think the Dodgers' trading Pedro to the Expos for Delino DeShields was worse, but the Dodgers only had one full season to go by. Montreal traded Pedro for Carl Pavano and Tony Armas, Jr. after he posted a 1.90 ERA, 305 Ks and a 17-8 record in 1997. Pedro continued to dominate with the Sox and helped break the Curse of the Bambino. Pavano had a couple of solid years, but wasn't an All-Star until joining Florida.

3. Giants trade Nathan and Liriano to Twins (2003)

The Giants thought catcher A.J. Pierzynski was the missing piece to a playoff run in 2003. The Giants sent Joe Nathan, Francisco Liriano and Boof Bonser to the Twins. Nathan has become one of the best closers in the game, and Liriano had an ERA of 2.16 in his first full season. He underwent Tommy John surgery but bounced back with a sub-4.00 ERA. Pierzynski is best known for being a cancer in the Giants' clubhouse.

4. Expos trade Randy Johnson to Mariners (1989)

The Expos traded away arguably the most dominant lefty pitcher of all time, Randy Johnson. Montreal dealt Johnson, Brian Holman and Gene Harris for Mark Langston and Mike Campbell. Langston was a solid pitcher, but the Big Unit went on to win five Cy Youngs, make 10 All-Star appearances, win more than 300 games, win a World Series, pitch a perfect game and rank 2nd all-time in strkeouts.

5. Red Sox trade Jeff Bagwell to Astros (1990)

At the time, this deal didn't look as lopsided as it turned out to be. The Red Sox traded prospect Jeff Bagwell to the Astros for 36-year-old RP Larry Andersen for their playoff push. Andersen was solid and helped Boston win the AL East, but he left after the season. Bagwell won the 1991 Rookie of the Year, won the 1994 NL MVP, hit 449 home runs and became one of the Astros' best all-time players.